Same issue on one of my W123's. Rather than pay >$300 new or $50 used, plus days tearing into the dash, I recirculated hot silicate stop leak, which I think worked. My son drives that car at college and doesn't communicate.

If in your position, I would look into installing a heater core from another car that is cheap and robust, like for a 60's Chevy. Indeed, since so hard digging into the dash, maybe something that would last forever like an after-market tube-fin transmission cooler. I would use all soldered copper tubing. Anyway, what I have pondered. I know W123's have a unique 3-tube Behr heater core as used in Semis, but it can be re-plumbed for 2 tubes. We don't deal with German winters in CA.